In process measurements and automation technology, for measuring physical parameters—e.g. mass flow, density and/or viscosity—of media flowing in pipelines—for instance, a medium in the form of an aqueous liquid, a gas, a liquid-gas mixture, a vapor, an oil, a paste, a slurry or other flowable material—such in-line measuring devices are often used, which, by means of a measuring transducer of vibration type flowed through by the medium and a measuring and operating circuit connected thereto, effect reaction forces in the medium, e.g. Coriolis forces corresponding to the mass flow, inertial forces corresponding to the density of the medium and/or frictional forces corresponding to the viscosity of the medium, etc., and derived from these, produce a measurement signal representing the respective mass flow, the respective viscosity and/or the respective density of the medium. Such measuring transducers, which are especially embodied as Coriolis mass flow meters or Coriolis mass flow/densimeters, are described at length and in detail in, for example, EP-A 1 001 254, EP-A 553 939, US-A 2002/0157479, US-A 2006/0150750, U.S. Pat. Nos. 5,370,002, 5,796,011, 6,308,580, 6,415,668, 6,711,958, 6,920,798, 7,134,347, 7,392,709, or WO-A 03/027616.
Each of the measuring transducers includes a transducer housing, of which an inlet-side, first housing end is at least partially formed by means of a first flow divider having exactly two circularly cylindrical or conical flow openings mutually spaced apart from one another, and an outlet-side, second housing end at least partially formed by means of a second flow divider having exactly two flow openings mutually spaced apart from one another. In the case of some of the measuring transducers shown in U.S. Pat. Nos. 5,796,011, 7,350,421, or US-A 2007/0151368, the transducer housing comprises a rather thick-walled, circularly cylindrical tube segment, which forms at least a middle segment of the transducer housing.
For conveying medium flowing at least at times, and, in given cases, also extremely hot, the measuring transducers furthermore comprise, in each case, exactly two, bent, measuring tubes, which are connected for parallel flow, which are made of metal, especially steel or titanium, and which are placed within the transducer housing and held oscillatably therein by means of the aforementioned flow dividers. A first of the measuring tubes (which are most often equally constructed and extend parallel to one another) opens with an inlet-side, first measuring tube end into a first flow opening of the inlet-side, first flow divider, and opens with an outlet-side, second measuring tube end into a first flow opening of the outlet-side, second flow divider, and a second of the measuring tubes opens with an inlet-side, first measuring tube end into a second flow opening of the first flow divider, and opens with an outlet-side, second measuring tube end into a second flow opening of the second flow divider. Each of the flow dividers includes, additionally, flanges with sealing surfaces for fluid-tight connecting of the measuring transducer to tube segments of the pipeline serving, respectively, for supplying medium to and for removing medium from the measuring transducer.
For producing the above-mentioned reaction forces, the measuring tubes are caused to vibrate during operation, driven by an exciter mechanism serving for producing or maintaining mechanical oscillations, especially bending oscillations, of the measuring tubes in the so-called driven or wanted mode. The oscillations in the wanted mode are most often, especially in the case of application of the measuring transducer as a Coriolis mass flow meter and/or densimeter, at least partially embodied as lateral bending oscillations, and in the case of medium flowing through the measuring tubes, as a result of Coriolis forces induced therein, are superimposed upon by additional, equal-frequency oscillations in the so-called Coriolis mode. Accordingly, the—here most often electro-dynamic—exciter mechanism is embodied in such a manner that, therewith, the two measuring tubes are differentially excitable—thus via entry of exciter forces acting simultaneously along a shared line of action, but in opposed directions—in the wanted mode at least partially, especially also predominantly, to opposite-equal bending oscillations.
For registering vibrations (especially bending oscillations excited by means of the exciter mechanism) of the measuring tubes, and for producing oscillation signals representing vibrations, the measuring transducers additionally in each case have a sensor arrangement, which reacts to relative movements of the measuring tubes, and is most often likewise electrodynamic. Typically, the sensor arrangement is formed by means of an inlet-side oscillation sensor registering oscillations of the measuring tubes differentially—thus registering only relative movements of the measuring tubes—as well as an outlet-side oscillation sensor also registering oscillations of the measuring tubes differentially. Each of the oscillation sensors, which are usually constructed equally to one another, is formed by means of a permanent magnet held on the first measuring tube and a cylindrical coil held on the second measuring tube and permeated by the magnetic field of the magnet.
In operation, the above-described tube arrangement formed by means of the two measuring tubes is excited by means of the electro-mechanical exciter mechanism at least at times in the wanted mode to execite mechanical oscillations at least one dominating, wanted, oscillation frequency. In such case, usually selected as the oscillation frequency for the oscillations in the wanted mode is a natural, instantaneous resonance frequency of the tube arrangement, which, in turn, is dependent essentially both on the size, shape and material of the measuring tubes, as well as also on an instantaneous density of the medium; in given cases, this wanted oscillation frequency can also be significantly influenced by an instantaneous viscosity of the medium. As a result of the fluctuating density of the medium to be measured and/or as a result of media changes performed during operation of the measuring transducer, the wanted oscillation frequency is variable at least within a calibrated—and in this respect predetermined—wanted frequency band, which correspondingly shows a predetermined lower and a predetermined upper limit frequency.
For defining a wanted oscillatory length of the measuring tubes and, in association therewith, for adjusting the wanted frequency band, measuring transducers of the above-described type additionally most often comprise: At least one inlet-side coupling element for forming inlet-side oscillation nodes for opposite-equal vibrations, especially bending oscillations, of the two measuring tubes, wherein this inlet-side coupling element is affixed, spaced from both flow dividers, to both measuring tubes; as well as at least one outlet-side coupling element for forming outlet-side oscillation nodes for opposite-equal vibrations, especially bending oscillations, of the measuring tubes, wherein this outlet-side coupling element is affixed to both measuring tubes and spaced both from the two flow dividers as well as also from the inlet-side coupling element. In the case of curved measuring tubes, the length of a section of a bend line of the respective measuring tube extending between the inlet-side and the outlet-side coupling element, consequently an imaginary center line of said measuring tube connecting the areal centers of gravity of all imaginary cross sectional areas of the respective measuring tube, corresponds, in such case, to the wanted oscillatory length of the measuring tubes. By means of the coupling elements belonging, in this respect, to the tube arrangement, also an oscillation quality factor of the tube arrangement, as well as also the sensitivity of the measuring transducer as a whole, can additionally be influenced in such a manner that, for a minimum required sensitivity of the measuring transducer, at least one minimum wanted oscillatory length is to be provided.
Development in the field of measuring transducers of vibration type has by this point reached such a state that modern measuring transducers of the described type can, for practical purposes, satisfy highest requirements as regard precision and reproducibility of measurement results for a broad spectrum of applications in the field of flow measurement technology. Thus, such measuring transducers are in practice used for mass flow rates of only some few g/h (gram per hour) up to some t/min (tons per minute), at pressures of up to 100 bar for liquids or even over 300 bar for gases. The accuracy of measurement achieved in such case usually lies, for instance, at 99.9% of the actual value, or more, and, respectively, a measuring error of, for instance, 0.1%, wherein a lower limit of the guaranteed measurement range can, by all means, lie, for instance, at 1% of the measurement range end value. Due to the great bandwidth of opportunities for use, industrial grade measuring transducers of vibration type are available with nominal diameters (corresponding to the caliber of the pipeline to be connected to the measuring transducer or the caliber of the measuring transducer measured at the connecting flange), which lie in a nominal diameter range of between 1 mm and 250 mm, and in the case of a maximum nominal mass flow rate of 1000 t/h, are specified respectively for pressure losses of less than 3 bar. A caliber of the measuring tubes lies, in such case, in a range between, for instance, 80 mm and 100 mm.
In spite of the fact that, by this point, measuring transducers are available for use in pipelines with very high mass flow rates and, in association therewith, very large calibers of far over 100 mm, there still exists significant interest to use measuring transducers of high precision and low pressure loss also for still larger pipeline calibers, for instance, of 300 mm or more, or mass flow rates of 1500 t/h or more, for instance, for applications in the petrochemical industry or in the field of transport and handling of petroleum, natural gas, fuels, etc. This leads in the case of correspondingly scaled enlargement of measuring transducer designs known and already established in the state of the art, especially from EP-A 1 001 254, EP-A 553 939, US-A 2002/0157479, U.S. Pat. Nos. 5,370,002, 5,796,011, 6,308,580, 6,711,958, 7,134,347, 7,350,421, or WO-A 03/027616, to geometric dimensions assuming exorbitantly high magnitudes, especially geometric dimensions due to the desired oscillation characteristics, the required load capacity as well as the maximum allowed pressure loss, especially the installed length corresponding a distance between the sealing surfaces of the two flanges and, in the case of curved measuring tubes, to a maximum lateral expanse of the measuring transducer. Associated therewith, the empty mass of the measuring transducer also increases unavoidably, wherein conventional measuring transducers of large nominal diameter are already implemented with an empty mass of, for instance, 400 kg. Investigations, which have been performed for measuring transducers with two bent measuring tubes—for instance, according to U.S. Pat. Nos. 7,350,421 or 5,796,011—as regards their scaled adapting to still greater nominal diameters, have, for example, had the result that, for nominal diameters of more than 300 mm, the empty mass of a conventional measuring transducer enlarged to scale would lie far over 500 kg, along with an installed length of more than 3000 mm and a maximal lateral expanse of more than 1000 mm.